Influenza is an acute viral infection that spreads easily from person to person. There are three types of seasonal influenza – A, B and C. Seasonal influenza is characterized by a sudden onset of high fever, cough (usually dry), headache, muscle and joint pain, severe malaise (feeling unwell), sore throat and runny nose. Most people recover from fever and other symptoms within a week without requiring medical attention. But influenza can cause severe illness or death in people at high risk. The incubation period is about two days.
Influenza epidemics occur yearly during autumn and winter in temperate regions. Illnesses result in hospitalizations and deaths mainly among high-risk groups (the very young, elderly or chronically ill). Worldwide, these annual epidemics result in about three to five million cases of severe illness, and about 250 000 to 500 000 deaths.
Most deaths associated with influenza in industrialized countries occur among people age 65 or older. In some tropical countries, influenza viruses circulate throughout the year with one or two peaks during rainy seasons.